1. Field of the Invention
The present invention relates to a vehicle control device and a vehicle control method for controlling a motion state of a vehicle, and particularly to control of the state of the vehicle based on the cornering power of wheels.
2. Description of the Related Art
There has been hitherto known a vehicle control method of controlling a motion state of a vehicle by controlling a driving/braking torque applied to the vehicle or controlling a suspension characteristic of the vehicle. According to this vehicle control method, for example, the motion state is controlled to be optimized under a traveling state such as cornering, thereby enhancing steering stability. As one of these techniques is known a vehicle control device for controlling the motion state of a vehicle by using a wheel frictional force utilization factor (for example, see Japanese Examined Patent Application No. 3132190). This vehicle control device determines the vehicle frictional force utilization factor for each wheel, and controls a wheel state of each wheel so that the vehicle frictional force utilization factor approaches a target vehicle frictional force utilization factor. The wheel frictional force utilization factor is calculated as a rate of the actual frictional force (a resultant force of a longitudinal force and a lateral force that actually occurs between the wheels and road surface) to the maximum frictional force (a product of an actual friction coefficient between the wheels and the road surface and vertical force actually occurring between the wheels and the road surface).
The inventor has found that it is preferable to pay attention to cornering power of wheels in order to effectively control the motion state of a vehicle under a traveling state (for example, cornering, traveling on road surface with a low friction coefficient, etc.) to which the above-described vehicle control is effective. This is because the cornering power indicates response of variation of the behavior of the vehicle (cornering force) in accordance with the magnitude of the value thereof, and thus it is a value having close relationship with the motion state of the vehicle. Accordingly, the steering stability of the vehicle can be estimated on the basis of the cornering power. For instance, formulation (for example, static margin, stability factor, etc.) is generally carried out using cornering power in steering stability analysis or vehicle motion control using a vehicle motion model considering only two degrees of freedom (the model considering the translation motion in the lateral direction and the rotational motion around the vertical axis).
The method disclosed in the Japanese Examined Patent Application No. 3132190 enhances the motion state of the vehicle by bringing the vehicle frictional force utilization factor closer to the target frictional force utilization factor. However, focusing on the cornering power of the wheels, there is a probability that the cornering power of the wheels has an improper value for each wheel even when the vehicle frictional force utilization factor of the wheels approaches the target friction utilizing factor. For example, the controllability of the vehicle may be lost when the wheel cornering power is reduced.